The Deltoid muscle is chosen as there are less blood vessels to hit when injecting.  Chances of an intravenous injection is low.

Lots of muscle mass to hold the fluid while it does the job.

Current injection techniques, according to some recent nursing grads, especially in children advocate landmarking and a rapid injection without aspiration.  Quick and less pain they are told.

In Dentistry, there is more vasculature in the oral region and a higher opportunity to inject into a vessel.   If one inadvertently  does inject into a vessel the epinephrine or equivalent has an immediate effect on the heart.  The patient notices an increased heart rate which is quite alarming to them.  It passes fairly quickly.  We deliver the fluid very slowly to allow it to disperse in the tissue.  A painless shot even after aspirating.  Most syringes we use are self-aspirating.

I’ve seen quite a few testimonials of those who experienced an immediate adverse reaction to the jab.  Some were long lasting. some not.  I assume they mainlined it.  We may never know what technique the vaccinators in the clinical study used.

Reaching out to those with Medical backgrounds to comment on this if possible.